Component: Developer Tools → Developer Tools: Console
Apart from the Browser Console & Browser Toolbox, we now have the Add-on Toolbox which you can get at from about:addons by clicking the "Debug" button. Doesn't that suffice or is it that you didn't know about these options?
I didn't know about it, but that doesn't help: the console there stays empty.
OK, that sounds like a bug in the Add-on Toolbox then (it may even be a dupe, but I can't find it right now). It is by design that the error doesn't appear in the web console.
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